1. Field of the Invention
This invention relates to thin film magnetic recording media and methods for forming thin film magnetic recording media.
2. Description of the Prior Art
It is known in the art that metallic thin films permit a higher recording density than conventional iron oxide particles suspended in a binder. In order to achieve a high recording density, it is necessary to minimize the film thickness, e.g., below 1000.ANG.. In addition, the coercivity Hc must be high enough to sustain a high density of flux reversals per inch. For example, to sustain a packing density of 10,000 to 50,000 flux reversals per inch, the coercivity is preferably between 600 and 2000 Oersteds (Oe). If the coercivity is greater than 2000 Oe, it is difficult to both read and write data onto the film using conventional single element read/write heads.
In order to provide a good output signal from the media, it is necessary to have a high saturation magnetization Ms, e.g. greater than 400 emu/cc and preferably greater than 800 emu/cc. It is also necessary to have a high magnetic remanent Mr and a hysteresis squareness S of at least 80%. (S=Mr/Ms.) The strength of the magnetic field received by the disk read head is proportional to the film thickness T times the magnetic remanent Mr. Mr. T should generally be greater than approximately 2.times.10.sup.-3 emu/cm.sup.2 in order to achieve strong enough output signal from the film.
The above-mentioned parameters are described in greater detail in "Thin Films for Magnetic Recording Technology: A Review" by J. K. Howard, published in the Journal of Vacuum Science and Technology in 1985, and incorporated herein by reference.
Typical films used in magnetic recording media include cobalt alloys such as Co-Ni, Co-Pt, and Co-Ni-Pt. A number of processes have been suggested for controlling the coercivity Hc, saturation magnetization Ms, and magnetic remanent Mr of such films. For example, a paper entitled "Thin-Film Memory Disc Development" by Opfer et al., published in the Hewlett-Packard Journal in 1985, incorporated herein by reference, suggests that the coercivity of a film can be controlled by forming a Co-Pt film on a chromium underlayer. The coercivity of the film is dependent on the thickness of the chromium underlayer. Unfortunately, manufacturing processes including the step of forming a chromium underlayer are relatively complicated and expensive.
Opfer also indicates that coercivity can be controlled by varying the platinum concentration in the film. However, this means that if it is desired to use the same sputtering equipment to form films of different coercivities, it is necessary to change the sputtering target, which is generally inconvenient to do.
Japanese patent application No. 171694/82, filed by Masahiro Yanagisawa on Sept. 30, 1982, incorporated herein by reference, suggests that coercivity of a Co-Pt-Ni film can be controlled by varying the amount of nickel in the film. Unfortunately, this technique also requires changing the sputtering target if it is desired to change the film coercivity.
"Effect of Nitrogen on the High Coercivity and Microstructures of Co-Ni Alloy Films" by H. Maeda, published in the Journal of Applied Physics in 1982, incorporated herein by reference, discusses controlling the coercivity of a Co-Ni film by sputtering the film in an atmosphere including argon and nitrogen. Maeda increases the coercivity of the film by increasing the nitrogen gas concentration in the sputtering chamber, e.g., to a concentration of about 24% by volume. Unfortunately, this causes the film saturation magnetization Ms to decrease.